Pulsed Optical Excitation Research Articles

Ultrafast photo-induced O2− channel-opening in oxygen vacancy ordered SrCoO2.5 film

The recent development of electric-field controlled brownmillerite SrCoO2.5 (BM-SCO) to SrCoO3-δ phase transformation greatly enriches the controlling diversity of functional materials. However, the required potential is much larger than that for the standard electrolysis of H2O and the detailed mechanisms for the corresponding oxygen insertion are still unclear. In this study, we mimic such electric-field control step with optical pulse excitation. In specific, by exciting BM-SCO thin film with femtosecond 400 or 800 nm pulses, and monitoring the lattice dynamics using ultrafast x-ray diffraction, we find that 400 nm photo-excitation can induce a distinctive transient BM-SCO state containing both Co2+ and Co4+, which is more suitable for O2− invasion. This transient BM-SCO state is suggested to originate from the redistribution of electrons on CoT (tetragonal layer) and CoO (octahedral layer) 3d orbitals, which is further confirmed by femtosecond transient reflectance measurements. We suggest that this distinctive transient BM-SCO state, which is critical for the phase transition, is also induced during the electric-field controlled BM-SCO to SrCoO3-δ phase transformation. This study intends to contribute an intriguing research thought for the inherent mechanism that might be powerless with traditional means and a special phase control method as well.

Emission dynamics of optically driven aluminum nitride quantum emitters

Aluminum nitride is a technologically important wide band-gap semiconductor which has been shown to host bright quantum emitters. We use photon emission correlation spectroscopy (PECS), time-resolved photoluminescence (TRPL), and state-population dynamic simulations to probe the dynamics of emission under continuous wave (CW) and pulsed optical excitation. We infer that there are at least four dark shelving states, which govern the TRPL, bunching, and saturation of the optical transition. We study in detail the emission dynamics of two quantum emitters (QEs) with differing power-dependent shelving processes, hypothesized to result from charge ionization and recombination. These results demonstrate that photon bunching caused by shelving the system in a dark state inherently limits the saturation rate of the photon source. In emitters where increasing optical power deshelves the dark states, we observe an increased photon emission intensity. Published by the American Physical Society 2024

Progress on Coherent Perovskites Emitters: From Light‐Emitting Diodes under High Current Density Operation to Laser Diodes

Perovskites exhibit appealing optical and electrical properties, making them attractive candidates for efficient luminescent materials with low‐cost and straightforward fabrication processes. Their versatility is highlighted by the ability to deposit perovskite thin films on various substrates, including silicon, glass, sapphire, and flexible substrates, enabling potential monolithic integration on silicon for applications such as photonic integrated circuits, high‐speed communication. Extensive studies on perovskite light‐emitting diodes have shown external quantum efficiencies exceeding 20% across a wide spectral range from deep blue to near‐infrared, with chirality. Additionally, perovskite‐based lasing action has been achieved under pulsed optical excitation and continuous‐wave operation, as well as in functional diode structures. However, realizing electrically driven perovskite laser diodes for practical applications requires the injection of intense current densities. This review provides a comprehensive overview of the historical progress in perovskite lasers and light‐emitting didoes, along with important design considerations essential for their development.

Room temperature quantum emitters in aluminum nitride epilayers on silicon

Room temperature quantum emitters have been reported in aluminum nitride grown on sapphire, but until now they have not been observed in epilayers grown on silicon. We report that epitaxial aluminum nitride grown on silicon by either plasma vapor deposition or metal-organic vapor phase epitaxy contains point-like emitters in the red to near-infrared part of the spectrum. We study the photon statistics and polarization of emission at a wavelength of 700–750 nm, showing signatures of quantized electronic states under pulsed and CW optical excitation. The discovery of quantum emitters in a material deposited directly on silicon can drive integration using industry standard 300 mm wafers, established complementary metal-oxide-semiconductor control electronics, and low marginal-cost mass-manufacturing.

Focusing Surface Acoustic Waves with a Plasmonic Hypersonic Lens.

Plasmonic nanoantennas have proven to be efficient transducers of electromagnetic to mechanical energy and vice versa. The sudden thermal expansion of these structures after an ultrafast optical pulsed excitation leads to the emission of hypersonic acoustic waves to the supporting substrate, which can be detected by another antenna that acts as a high-sensitivity mechanical probe due to the strong modulation of its optical response. Here, we propose and experimentally demonstrate a nanoscale acoustic lens comprised of 11 gold nanodisks whose collective oscillation at gigahertz frequencies gives rise to an interference pattern that results in a diffraction-limited surface acoustic beam of about 340 nm width, with an amplitude contrast of 60%. Via spatially decoupled pump-probe experiments, we were able to map the radiated acoustic energy in the proximity of the focal area, obtaining a very good agreement with the continuum elastic theory.

Efficient fabrication of high quality SU-8 photoresist based microsphere lasers via emulsion

SU-8 photoresist is a highly important material in the field of microfabrication and photonics owing to its low cost, excellent chemical and mechanical durability, high refractive index and transparency in the visible range. As a result, SU-8 photoresist has been employed as a cavity matrix for microsphere lasers. However, the current fabrication technique of SU-8 based microsphere lasers is complex and time-consuming. Here, we demonstrate a novel, cost-effective fabrication method for dye-doped SU-8 microspheres with diameters ranging from about 15–100 µm. These microspheres exhibit efficient lasing emission under optical pulse excitation. Lasing thresholds of 20–30 µJ mm−2 and quality factors ranging from 1500 to 3000 are achieved. The size dependence of lasing characteristics indicates that the lasing mechanism is due to whispering gallery mode. Interestingly, these microsphere lasers can work in water, presenting promising application prospects in the fields of biological and chemical sensors.

Large-field-of-view optical-resolution optoacoustic microscopy using a stationary silicon-photonics acoustic detector.

Optical-resolution optoacoustic microscopy (OR-OAM) enables label-free imaging of the microvasculature by using optical pulse excitation and acoustic detection, commonly performed by a focused optical beam and an ultrasound transducer. One of the main challenges of OR-OAM is the need to combine the excitation and detection in a coaxial configuration, often leading to a bulky setup that requires physically scanning the ultrasound transducer to achieve a large field of view. The aim of this work is to develop an OR-OAM configuration that does not require physically scanning the ultrasound transducer or the acoustic beam path. Our OR-OAM system is based on a non-coaxial configuration in which the detection is performed by a silicon-photonics acoustic detector (SPADE) with a semi-isotropic sensitivity. The system is demonstrated in both epi- and trans-illumination configurations, where in both configurations SPADE remains stationary during the imaging procedure and only the optical excitation beam is scanned. The system is showcased for imaging resolution targets and for the in vivo visualization of the microvasculature in a mouse ear. Optoacoustic imaging with focal spots down to , lateral resolution of , and a field of view higher than 4mm in both lateral dimensions were demonstrated. We showcase a new OR-OAM design, compatible with epi-illumination configuration. This setup enables relatively large fields of view without scanning the acoustic detector or acoustic beam path. Furthermore, it offers the potential for high-speed imaging within compact, miniature probe and could potentially facilitate the clinical translation of OR-OAM technology.

Large Tolerance of Lasing Properties to Impurity Defects in GaAs(Sb)‐AlGaAs Core‐Shell Nanowire Lasers

AbstractGaAs‐AlGaAs based nanowire (NW) lasers hold great potential for on‐chip photonic applications, where lasing metrics have steadily improved over the years by optimizing resonator design and surface passivation methods. The factor that will ultimately limit the performance will depend on material properties, such as native‐ or impurity‐induced point defects and their impact on non‐radiative recombination. Here, the role of impurity‐induced point defects on the lasing performance of low‐threshold GaAs(Sb)‐AlGaAs NW‐lasers is evaluated, particularly by exploring Si‐dopants and their associated vacancy complexes. Si‐induced point defects and their self‐compensating nature are identified using correlated atom probe tomography, resonant Raman scattering, and photoluminescence experiments. Under pulsed optical excitation the lasing threshold is remarkably low (<10 µJ cm−2) and insensitive to impurity defects over a wide range of Si doping densities, while excess doping ([Si]>1019 cm−3) imposes increased threshold at low temperature. These characteristics coincide with increased Shockley‐Read‐Hall recombination, reflected by shorter carrier lifetimes, and reduced internal quantum efficiencies (IQE) . Remarkably, despite the lower IQE the presence of self‐compensating Si‐vacancy defects provides an improved temperature stability in lasing threshold with higher characteristic temperature and room‐temperature lasing. These findings highlight an overall large tolerance of lasing metrics to impurity defects in GaAs‐AlGaAs based NW‐lasers.

Modeling of Charge Injection, Recombination, and Diffusion in Complete Perovskite Solar Cells on Short Time Scales.

A model of charge population decay upon ultrafast optical pulse excitation in complete, working perovskite solar cells is proposed. The equation, including charge injections (extractions) from perovskite to contact materials, charge diffusion, and charge recombination via first-, second-, and third-order processes, is solved using numerical simulations. Results of simulations are positively verified by broadband transient absorption results of mixed halide, triple-cation perovskite (FA0.76MA0.19Cs0.05Pb(I0.81Br0.19)3). The combined analytical and experimental findings reveal the best approaches for the proper determination of the crucial parameters that govern charge transfer dynamics in perovskite solar cells on picosecond and single nanosecond time scales. Measurements from both electron and hole transporting layer sides under different applied bias potentials (zero and close to open circuit potential) and different pump fluence (especially below 5 μJ/cm2), followed by fitting of parameters using numerical modeling, are proposed as the optimal methodology for describing the processes taking place in efficient devices.

Nuclear electric resonance for spatially resolved spin control via pulsed optical excitation in the UV-visible spectrum

Nuclear electric resonance for spatially resolved spin control via pulsed optical excitation in the UV-visible spectrum

Ultrafast snapshots of terahertz electric potentials across ring-shaped quantum barriers

Abstract Probing the time evolution of the terahertz electric field within subwavelength dimensions plays a crucial role in observing the nanoscale lightwave interactions with fundamental excitations in condensed-matter systems and in artificial structures, such as metamaterials. Here, we propose a novel probing method for measuring terahertz electric potentials across nanogaps using a combination of optical and terahertz pulse excitations. To achieve this, we employ ring-shaped nanogaps that enclose a metallic island, allowing us to capture tunneling charges when subjected to terahertz electromagnetic pulse illumination. By controlling and manipulating the terahertz tunneling charges through a focused optical gate pulse, we can obtain the terahertz potential strength as a function of spatial coordinates and time delays between pulses. To accurately quantify the time evolution of terahertz electric potential across quantum barriers, we carefully calibrate the recorded nonlinear tunneling current. Its on-resonance and off-resonance behaviors are also discussed, providing valuable insights into the antenna’s characteristics and performance.

Parallel Photoelectron Storage and Visual Preprocessing Based on Nanowire Defect Engineering for Image Degradation

AbstractThe image degradation owing to the absorption and scattering of optical propagation medium makes the non‐visualization and anamorphose in real image. The complex algorithms based on highly integrated hardware can restore degraded image while it causes inefficient serial operation and storage redundancy. One improved strategy is to parallel functional integration in front‐end visual perceptron. Here, a parallel photoelectron storage and visual preprocessing in nanowire perceptron to synchronously achieve image perception, visual memory, and in‐sensor preprocessing are demonstrated. This functional integration is originated from charge‐resolved storage in temporal or frequency domain under optical pulsed excitation due to cascaded defect engineering. The proposed system enhances the peak signal‐to‐noise ratio (PSNR) of degraded image from 152 to 181 dB by feature decompression and also has 7.2% of PSNR improvement by noise filtration. This system is validated to improved train accuracy of back‐end artificial neural network (ANN) by 32.8% and shorten its iteration period by 77.4%.

Pixelated vertical-cavity surface-emitting laser arrays from colloidal quantum dot films

Pixelated arrays of vertical-cavity surface-emitting lasers from colloidal quantum dot thin films have been demonstrated. CdSe/ZnCdS core/shell structured colloidal quantum dots have been used as laser gain medium. 10 pairs of alternating layers between GaN and nanoporous GaN consisted highly reflective distributed Bragg reflectors, exhibiting more than 99% of reflectivity in the spectral range of 600 nm–650 nm. Close-packed colloidal quantum dot solid thin films were sandwiched between the two GaN/nanoporous GaN distributed Bragg reflectors to achieve well-defined lasing outputs under pulsed optical excitation with the lasing threshold as low as 11.0 μJ/cm2. The resulting CQD-VCSEL devices exhibited a lasing output power of 1.5 μW and a conversion efficiency of 11.4%. Finally, a two-axis scanning micro-mirror was used to complete the 25 (5 by 5) pixelated laser arrays system.

Nanomechanics with plasmonic nanoantennas: ultrafast and local exchange between electromagnetic and mechanical energy

Converted into mechanical nanoresonators after optical pulsed excitation and electron decay into coherent acoustic phonons, plasmonic nanoantennas produce a periodic modulation of their optical properties, allowing, in turn, an optical reading of these extremely small movements. In this work, we review the physics of these nanoresonators and their acoustic vibrations, whose frequencies are in the range of a few to tens of GHz. The accurate determination of their oscillation frequencies allows them to act as mechanical nanoprobes, measure local mechanical moduli of the environment, and perform high-resolution imaging using phononic reconstruction. Furthermore, the internal and external damping mechanisms that affect the quality factor of the nanoresonator and, in particular, the role of the substrate when the nanoantennas are integrated into platforms and probed individually are also reviewed. Finally, we discuss the all-optical generation of hypersonic surface acoustic waves with nanoantennas and the importance of their manipulation for potential acousto-plasmonic devices operating in the GHz range and at nanoscale.

Onset of vortex clustering and inverse energy cascade in dissipative quantum fluids

Turbulent phenomena are among the most striking effects that both classical and quantum fluids can exhibit. Although classical turbulence is ubiquitous in nature, the observation of quantum turbulence requires the precise manipulation of quantum fluids such as superfluid helium or atomic Bose–Einstein condensates. Here we demonstrate the turbulent dynamics of a two-dimensional quantum fluid of exciton–polaritons, hybrid light–matter quasiparticles, both by measuring the kinetic energy spectrum and showing the onset of vortex clustering. We demonstrate that the formation of clusters of quantum vortices is triggered by the increase of the incompressible kinetic energy per vortex, showing the tendency of the vortex-gas towards highly excited configurations despite the dissipative nature of our system. These results lay the basis for investigations of quantum turbulence in two-dimensional fluids of light. The turbulent dynamics of a 2D quantum fluid of exciton–polaritons is measured in a planar AlGaAs microcavity after a pulsed optical excitation. Clear evidence of both the onset of vortex clustering and inverse energy cascade is provided.

Ultrafast Dynamics of Intrinsic Anomalous Hall Effect in the Topological Antiferromagnet Mn_{3}Sn.

We investigate ultrafast dynamics of the anomalous Hall effect (AHE) in the topological antiferromagnet Mn_{3}Sn with sub-100fs time resolution. Optical pulse excitations largely elevate the electron temperature up to 700K, and terahertz probe pulses clearly resolve ultrafast suppression of the AHE before demagnetization. The result is well reproduced by microscopic calculation of the intrinsic Berry-curvature mechanism while the extrinsic contribution is clearly excluded. Our work opens a new avenue for the study of nonequilibrium AHE to identify the microscopic origin by drastic control of the electron temperature by light.

Time-Domain Modelling of Pulsed Photoconducting Sources—Part II: Characterization of an LT GaAs Bow-Tie Antenna

Drude’s description of the response of low-temperature gallium arsenide to optical pulse excitation is used to evaluate the components of a time-domain Norton equivalent circuit of a photoconductive antenna (PCA) source. The saturation of the terahertz (THz) radiated power occurring at large optical excitation levels was previously associated by the scientific community to radiation and charge screening of the bias. With the present circuit, we are able to model accurately the measured saturation as only due to the EM feedback from the antenna to the bias. The predicted THz radiated power is shown to match very accurately the measurements when the circuit is combined with an accurate description of the experimental conditions and the modeling of the THz quasi-optical (QO) channel.

Energy-band echoes: Time-reversed light emission from optically driven quasiparticle wave packets

The at-will control of quantum states is a primary goal of quantum science and technology. The celebrated Hahn echo exemplifies such quantum-state control based on a time-reversal process in a few-level system. Here, we propose a different echo phenomenon associated with the energy-band structure in quantum many-body systems. We show that the dynamics of quasiparticle wavepackets can be reversed by a driving electric-field pulse, yielding echoes with the time-reversed waveform of the optical excitation pulse when the quasiparticles recombine. The present echoes are observed not only in band insulators but also in correlated insulators, including a Mott insulator and a spontaneously-broken-symmetry charge-ordered insulator, in one- and higher-dimensional systems, irrespective of the integrability of the models. Analytical expressions reveal the conditions under which the echoes appear, and they also indicate that the frequency of the echo pulses reflects the dispersion relation for quasiparticles such as electron-hole pairs, doublon-holon pairs, and kink-antikink pairs. These findings provide a framework for all-optical momentum-resolved spectroscopy of the quasiparticles in quantum many-body systems.

Optically controllable magnetism in atomically thin semiconductors.

We report evidence that ferromagnetic order in electrostatically doped, monolayer transition metal dichalcogenide (TMD) semiconductors can be stabilized and controlled at zero magnetic field by local optical pumping. We use circular dichroism (CD) in reflectivity from excitonic states as a spatially resolved probe of charge-carrier spin polarization. At electron densities ne ~ 1012 cm−2, a diffraction-limited, circularly polarized optical pump breaks symmetry between oppositely polarized magnetic states and stabilizes long-range magnetic order, with carrier polarization exceeding 80% over an 8 μm by 5 μm extent. In time-resolved measurements with pulsed optical excitation, we observe that magnetic interactions amplify the initial pump-induced spin polarization by more than an order of magnitude. The optical control of magnetism with local optical pumps will unlock advancements in spin and optical technologies and provides a versatile tool in the study of correlated phases in two-dimensional electron gases.

Two-dimensional space–time terahertz memory in bulk SrTiO3

Polarizable materials with ultrafast responses are of great interest for the development of new sensors and memories under the influence of an electromagnetic field. Recent research efforts have demonstrated the role of optical excitation and intense terahertz (THz) pulses in inducing an ultrafast polar order and revealing a hidden phase transition in S r T i O 3 (STO), respectively. Here we show that the surfaces of STO crystals at room temperature act as ultrafast sensors that enable sub-picosecond switching through the Kerr effect and recording of polar THz intensity with spatial resolution below the diffraction limit through THz-field-induced dipole alignment followed by multi-picosecond relaxation time recovery. The contrast sensitivity and spatial resolution achieved by the STO sensor are significantly superior to those of present-day near-field THz sensors based on the linear Pockels effect, and more importantly, its ability to remain polarized for several picoseconds opens the door to a new strategy for building an ultrafast space–time THz memory.